1. Field of the Invention
The present invention relates to integrated circuits and particularly to an integrated circuit including a bipolar transistor and a hetero bipolar transistor.
2. Description of the Related Art
Bipolar transistors, i.e. transistors made of semiconductor material homogeneous with respect to the material, are known and include a collector, a base, and an emitter. In the case of an npn transistor, the collector is formed of an n-doped semiconductor material, the base is formed of a p-doped semiconductor material, and the emitter is formed of an n-doped semiconductor material.
A species of such an npn bipolar transistor (BT) is the so-called double poly-silicon bipolar transistor in self-aligning technology.
What is further known in the art is the hetero bipolar transistor (HBT), which is more critical in the technological production than the conventional bipolar transistor. A hetero bipolar transistor is also an npn or pnp structure, the base material, however, being a semiconductor material other than the material of the emitter and the collector. If, for example, a SiGe hetero bipolar transistor is mentioned, this usually means that the substrate, the collector and the emitter of the HBT are formed of silicon, while the base is formed of silicon germanium. An example for a silicon-germanium hetero bipolar transistor (HBT) is the double poly SiGe HBT in self-aligning technology with selectively epitaxially grown base.
What is further known in the art is the so-called BiCMOS technology, in which, on an integrated circuit, conventional bipolar transistors are integrated with CMOS inverters, wherein a CMOS inverter consists of an n-channel FET and a p-channel FET.
Classic bipolar transistors show a good yield in the production, i.e. the production or manufacturing steps for classic bipolar transistors are controlled well. The disadvantage of classic bipolar transistors compared to hetero bipolar transistors is the fact that they produce more noise than hetero bipolar transistors and that they have lower cut-off frequencies. Particularly for low-noise high-frequency applications, a hetero bipolar transistor is therefore preferred to a normal bipolar transistor.
On the other hand, a classic bipolar transistor has a good matching behavior due to the well-controllable technology. If, for example, a current mirror is to be constructed of transistors, two transistors should be as identical as possible, i.e. adapted or matched well to each other. In other words, the matching behavior of these two transistors should be as high as possible. It is known from the integrated circuit technology that, in the case of a less favorable matching behavior, as it occurs in hetero bipolar transistors, this less favorable matching behavior may be compensated by increasing the overall size of the transistors, which results in a higher chip area consumption. Expressed in other words, circuits of bipolar transistors can therefore be integrated smaller and saving more chip area. What is more important, however, is the disadvantage of the HBP that the yield, i.e. the number of functional transistors per waver, is significantly reduced compared to the yield with classic bipolar transistors.
Particularly with transmit/receive modules for portable telephones for other radio applications, there need to be especially low-noise and fast transistors at certain locations, such as in the first stage of the HF input amplifier. On the other hand, the noise property of the transistor will not be so important at other locations in this integrated transmitter/receive module, but rather the requirement that several transistors may be designed as identical as possible, such as in a symmetrical mixer for down-mixing the HF receive signal into the baseband. In addition, speed and/or cut-off frequency of the transistor are not of substantial importance in the baseband. What is of more importance here is a reliable characteristic for the behavior of the transistor and also, of course, the chip area consumption and, last but not least, the costs.
The costs for a single module, however, increase with decreasing yield, so that a production of a transmit/receive module using classic bipolar transistors alone may have good yield and a low chip area consumption, but falls behind with respect to cut-off frequency and noise behavior compared to a complete implementation of the module with HBTs, while the HBT integration causes higher chip area consumption and lower yield, i.e. higher costs.